This invention relates to the recovery of fluoride values from waste cathodes and materials used for the lining of aluminum electrolytic cells. More specifically, this invention relates to an improved method for the recovery of valuable components from reduction cell linings and cathodes.
The cathodes of electrolytic furnaces used in the production of aluminum are lined with carbon block and carbon lining paste compositions which are electrically conductive. During electrolytic operations, the carbon linings and cathodes gradually absorb bath materials, such as cryolite (Na.sub.3 AlF.sub.6), aluminum fluoride (AlF.sub.3), calcium fluoride (CaF.sub.2), alumina, and metallic aluminum. This absorption, principally of cryolite, frequently doubles the cathode weight of a cell during operation. When it becomes necessary to remove the spent cathodes and pot linings, a considerable amount of very expensive fluoride materials is involved. At the present time, due to the lack of feasible recovery systems, most of this spent or waste cell lining material is merely stockpiled, presenting environmental concerns relative to the possible leaching of fluoride salts into ground waters. In addition, the value of the fluoride materials involved, if suitably recoverable, is substantial.
Prior art methods for recovering cryolite from spent cathode materials have included extraction by sodium hydroxide, sodium carbonate, or water. U.S. Pat. Nos. 1,871,723 and 2,732,283 teach the treatment of carbon cell lining material with aqueous caustic solutions to yield sodium fluoride and sodium aluminate, which solutions may be processed to precipitate cryolite. U.S. Pat. No. 3,106,448 teaches reaction between fluoride values in spent liner and a water soluble carbonate to produce water soluble sodium fluoride, which may in turn be precipitated with sodium aluminate to form a cryolite. In addition, the extraction and recovery of alumina and fluoride values with dilute ammonia solutions is known.
One of the more recent methods for recovery of fluoride and aluminum values involves the pyrohydrolysis of the carbonaceous material, preferrably in a fluidized bed reactor. Pyrohydrolysis involves contacting the spent cathode and/or cell lining with water or steam at high temperatures, whereby the water introduced reacts with the fluoride compounds to form HF. However, it has been found that while the pyrohydrolysis of aluminum fluoride is relatively easy, calcium fluoride and, particularly, sodium fluoride are more difficult to react. U.S. Pat. Nos. 4,113,832, 4,158,701, 4,160,808, and 4,160,809 all relate to pyrohydrolsis techniques for the recovery of fluoride values from spent cell linings. However, these references fail to provide a feasible and economic method, due to exceptionally high temperature and excessive steam requirements. It has been found that temperatures in excess of 1100.degree. C. are required for the recovery of fluoride from cathode waste by pyrohydrolysis. Fluoride recovery is enhanced by increasing temperature, increasing exposure time, and by the use of considerable quantities of steam.
Thus, it may be observed that there is a well defined need for a suitable fluoride recovery technique, whereby more efficient and economical results may be obtained. It is a purpose of the present invention to provide such a method.
It is a further purpose of the present invention to provide a method for the treatment of cathode waste materials which efficiently decomposes the fluorine-containing components therein and provides for their efficient recovery in the form of aluminum trifluoride, and/or cryolite.